The efficiency of gas turbine engines is, to a great degree, dependent on the clearance between the free ends of the rotating component and the surrounding casing. To substantially decrease clearance losses between the rotor blades and the casing, a variety of abradable linings have been employed, so that the rotating component can cut a groove into the abradable lining with minimal damage to the rotor blades. For low temperature applications, such as exist at the inlet sections of a gas turbine engine, the abradable lining can be constructed from a variety of rubber-like materials to provide the desired tight operating clearance. However, as stage temperatures increase, such as those present in the compressor and turbine sections of the engine, rubber-like materials can no longer be employed. For such higher temperature applications (i.e., greater than 300.degree. and generally greater than 500.degree. C.) high melting point materials, such as metals, cermets or ceramics must be employed. Dense (non-porous) metals, cermets, or ceramics can not provide the desired degree of abradability, since the amount of energy required to cut a groove into the dense material and the amount of wear on the blade tips would be much too great. Therefore, the art has generally resorted to various low density metallic or ceramic linings to provide the requisite abradability.
Abradable metal linings are generally, (i) sprayed onto the casing or (ii) bonded, e.g. by brazing, to the casing. The latter abradable liner materials are produced from rigid, skeleton-like matrix structures, i.e, structures having pores therein, such as honeycomb, or sinter bonded particulate metals (powders or fibers).
As noted above, a low density, generally porous material must be employed to permit the rotating component to wear into the liner material without incurring unduly high rub energy losses and unduly high wear of the rotating component. Such porous liners will generally exhibit density ratios (ratio of the density of the structure to the absolute density of the metal used in the structure--expressed in percent) of about 15 to about 35%. An unfortunate corollary to the use of porous materials is their high permeability to gas flow--resulting in performance losses, i.e., leakage between the blade tip and the pores of the abradable liner. To reduce such leakage the art has attempted to use metallic powders to fill at least a portion of the pores. Examples of these attempts are shown in U.S. Pat. Nos. 3,844,011 and 3,519,282. The products produced by either of these patented processes are subject to a similar deficiency, i.e., substantially decreased leakage is only achieved by using unduly large amounts of metal powders--which results in a high density liner and the poor rub characteristics (abradability) associated therewith. For example, it was found that the infiltration of a metal fiber structure (analogous to the one shown in the '282 patent) with sufficient metal powders to increase the density ratio from 21 to 25% provided only a slight improvement (decrease) in leakage, coupled with a significant deterioration of abradability characteristics.
Notwithstanding the rather meager improvement in decreased leakage achieved by metal powder impregnation, this technique, for practical considerations, is generally limited to abradable liner materials with comparatively large pores. The sinter-bonded, metal fiber products most commonly employed in gas turbines are made of extremely fine diameter fibers--with pore diameters generally below 40 microns. Such products are difficult to completely impregnate, even with dispersions of the finest metal particles (approx. 4 microns) presently commercially available.